If you would like to spend a few more dollars you can fix the screen easily, with no disassemble of the glass from the frame, which is the hard and time consuming part, you can purchase the screen and frame together for around $50.00 -$65.00 Pre-installed Kit : Apple iPhone 3G Lcd Glass Screen Cover + Touch Screen Digitizer + Frame + Home Flex Button + Ear Speaker + Tool + Cleaning Cloth
A few additional notes if you decide to go with the $20.00 option;
1) When you remove the screws, and pull the glass away from the case using the suction cup, BE GENTLE! The LCD and glass screen are attached to three connectors you will need to disconnect. You will see this step in the videos.
2) When you attempt to remove the glass from the LCD, after the inner screws are removed, GENTLY pry the glass screen frame away from the LCD and don't pry between the glass and LCD, you should be able to pry the two easily away on the side.
3) After you heat the glass to remove the broken glass from the frame, be GENTLE removing around the Home button.
4) Be sure your kit contains 3M tape for the Glass, Use my link to the top left of the post and the kit will contain 3M tape.
5) Finally you may need to purchase a small Philips head screw driver if the one in the kit doesn't fit the screws properly.
Good Luck! And you will be excited when you realize how much $$ you saved your self by doing this.